One known example of a photovoltaic device used in a solar cell that converts the energy from sunlight into electrical energy is a thin-film silicon-based photovoltaic device comprising a photovoltaic layer formed by using a plasma enhanced CVD method or the like to deposit thin films of a p-type silicon-based semiconductor (p-layer), an i-type silicon-based semiconductor (i-layer) and an n-type silicon-based semiconductor (n-layer).
Advantages of thin-film silicon-based solar cells include the comparative ease with which the surface area can be increased, and the fact that the film thickness is approximately 1/100th that of a crystalline solar cell, meaning minimal material is required. As a result, thin-film silicon-based solar cells can be produced at lower cost than crystalline solar cells. However, drawbacks of thin-film silicon-based solar cells include lower conversion efficiency than that of crystalline solar cells, and a slow deposition rate, resulting in poor productivity.
A tandem-type silicon-based solar cell in which a photovoltaic layer comprising an i-layer composed of amorphous silicon and a photovoltaic layer comprising an i-layer composed of crystalline silicon are stacked together is effective in improving the conversion efficiency of a thin-film silicon-based solar cell. The crystallinity of the crystalline silicon i-layer and the conversion efficiency of the solar cell are closely correlated, and it is known that the highest conversion efficiency is achieved when the crystalline silicon i-layer exhibits crystallinity that is close to the boundary between amorphism and crystallinity.
The crystallinity of the crystalline silicon i-layer is dependent upon the film deposition conditions such as the substrate temperature during deposition and the hydrogen dilution ratio. The crystallinity is represented, for example, by the ratio 1c/1a in the Raman spectrum between the crystalline silicon peak intensity at 520 cm−1 and the amorphous silicon peak intensity at 480 cm−1. Patent citation 1 discloses a process for producing a solar cell having high conversion efficiency by depositing the crystalline silicon i-layer under conditions where the substrate temperature Tsub and the ratio 1c/1a satisfy the relational formula 700≦Tsub×1c/1a≦1600.
One technique for improving the mass productivity of solar cells involves forming a crystalline silicon i-layer of uniform crystallinity and thickness to maintain solar cell performance, while increasing the deposition rate. Further, in another technique, a solar cell module is produced using a large surface area substrate. Generally, in the production of large surface area modules using a plasma enhanced CVD method, the crystallinity and film thickness of the crystalline silicon i-layer tend to exhibit distributions within the substrate plane that correspond with the gas flow and the discharge distribution. On the other hand, in order to improve the mass productivity of the solar cell, a larger RF power must be supplied to the plasma electrode to increase the deposition rate of the crystalline silicon i-layer. However, this also increases the plasma discharge distribution within the substrate plane, causing a broadening of the crystallinity and film thickness distributions within the substrate plane. As a result, in the case of a large surface area substrate, depositing the crystalline silicon i-layer uniformly across the substrate plane in order to achieve a level of crystallinity that yields superior performance for the solar cell containing the crystalline silicon i-layer is problematic, and achieving a combination of a high deposition rate and high performance is extremely difficult.
One technique for depositing a crystalline silicon layer of uniform crystallinity and film thickness on a large surface area substrate has been reported in non-patent citation 1, and comprises the deposition of a crystalline silicon layer using a short-pulsed plasma CVD method. In a short-pulsed CVD method, by pulsing an excited plasma, the spatial non-uniformity of the applied electric field during the plasma ON phases is moderated and the spatial distribution of the active species becomes more uniform, leading to improvements in the uniformity of the crystallinity and the film thickness. As a result, a large surface area solar cell having high conversion efficiency can be realized.
Patent Citation 1: Publication of Japanese Patent No. 3,943,080.
Non Patent Citation 1: Y. Fujioka et al., “Large scale, high-efficiency thin-film silicon solar cells fabricated by short-pulsed plasma CVD method”, PVSEC14th (Bangkok, 2004 January).